The altitude of the zero isotherm corresponds to the boundary between the liquid phase and the solid phase of the atmospheric water. When a frozen hydrometeor (that is, a particle of water, in this case frozen) passes through this boundary, it starts to progressively melt.
It is particularly important for a pilot to know where the zero isotherm is located. This is because the risks need to be understood differently depending on whether the liquid or frozen phase applies, because the risks that are run are not the same. Moreover, the processing operations performed on the data also depend on the phase.
It should be noted that, beyond the zero isotherm, the dielectric properties of the hydrometeor are modified, as is its reflectivity. It will be recalled that, for a given point, the reflectivity corresponds to the ratio of the energy reflected by this point to the total incident energy.
This rise in reflectivity is translated into the appearance of a bright strip in high-resolution radar images. However, radars on board carriers have a resolution that is too low for it always to be possible to view this bright strip.
One known solution for determining the zero isotherm is to use temperature sensors, on board the carrier (U.S. Pat. No. 6,741,203 and U.S. Pat. No. 6,201,494) associated with a temperature decrease model and a measurement of the altitude of the carrier. However, the use of a temperature indication gives imprecise results. For example, the measurements can be very sensitive to the degree of humidity of the atmosphere.
Moreover, this solution is applicable only in relatively low latitude regions for which the altitude of the carrier is situated under the tropopause. Indeed, a reversal of the trend of the temperature is observed beyond this boundary: the model used is therefore no longer valid.
Other methods involve using data transmitted by sources external to the carrier: for example, weather bulletins transmitted by ground or other stations (U.S. Pat. No. 6,424,288). However, the transmission of the information is not always possible to fleet airplanes.